The present invention relates to a method for controlling a hybrid traction assembly of a vehicle. Besides, the present invention relates to an automotive vehicle, especially an industrial plug-in hybrid vehicle, which can be controlled according to such a method.
A prior art hybrid vehicle usually has a hybrid traction assembly which can comprise an internal combustion engine, at least one electric reversible machine for performing traction of the hybrid vehicle and for generating electric regenerative energy when the vehicle slows down, and an electric energy storage device comprising, if it is of the co-called “plug-in” type, a connector for connection to the grid. Sometimes, instead of a reversible machine, a prior art hybrid vehicle can comprise an electric motor and an electric generator.
In a parallel type hybrid, both the internal combustion engine and the electric machine may provide torque to the driveline of the vehicle. The electric machine retrieves or delivers electric energy from/to an electric energy storage device which may comprise a set of electrochemical batteries, super-capacitors, etc.
In a series type hybrid, the internal combustion engine is not mechanically coupled to the vehicle driveline and delivers mechanical power to a first electrical machine which is mainly used as a generator. The electric energy produced by the generator is fed to one or several second electrical machine(s), preferably reversible. Said second electric machine is mechanically connected to the vehicle driveline so as to deliver driving torque to the vehicle driveline when used as a motor during motoring phases or to retrieve torque from the driveline when used as a generator during braking phases. Both electrical machines retrieve or deliver electric energy from/to at least one common electrical energy storage device which may comprise electrochemical batteries, super-capacitors, etc.
Other types of hybrids are known such as the so-called series-parallel where a power split device splits the torque from the engine between the vehicle driveline and a generator, while an electric machine is also connected to the vehicle driveline
The vehicle may comprise one or several electronic control units adapted to control the internal combustion engine and the electrical machine(s) in order to control directly or indirectly the amount of torque delivered or retrieved by each of internal combustion engine and electric machine(s). Electronic control units can thereby control the speed and/or the torque of the internal combustion engine and can also control the electrical power retrieved/delivered by the electric machine(s) from/to the electric energy storage device. It is quite common in the field of hybrid vehicles to have an electronic control unit which may implement several control strategies. Some of these strategies can be centred on seeking to maintain the state of charge of the electric energy storage device within a predefined window around a state of charge target. The state of charge target or target window can depend on various parameters, including vehicle speed, vehicle weight, battery state of health, etc. One easy way to modify at least partly the strategy is to change the state of charge target or target window and/or to change the way it is determined. For example, if the state of charge target is defined by a formula depending on a certain number of parameters, a change of strategy can be obtained by giving a fixed predetermined value to the state of charge target or target window, or by modifying the formula by which it is determined. In any case, modifying a state of charge target or a state of charge target window will be equivalent.
Some electric hybrid vehicles, which can be of the series, parallel or series-parallel type, are of the so-called plug-in type, meaning that their electrical energy storage device can also be charged when the vehicle is stopped by being connected to an external source of electric energy, such as the power grid. Such plug-in vehicles typically have an electrical energy storage device of larger capacity, mainly based on a set of electrochemical batteries.
A prior art controlling method for controlling such a plug-in hybrid electric vehicle usually comprises:
a connection phase for charging the battery set at a high level before the beginning of a vehicle mission;
a high rate depleting phase where the battery set delivers electric traction energy at the highest possible depletion rate; and
a sustaining phase where the state of charge is maintained in a sustaining range.
The high rate depleting phase corresponds to the fact that the battery set discharge or depletion is promoted at a discharge rate which is preferably close to the maximum possible discharge rate for the battery set at the instant operating conditions. For example, the electronic control unit can control the vehicle so that the discharge rate is, if possible, above 80% of the available discharge rate. In practice, the vehicle is controlled so that its required traction energy is delivered to the maximum possible extent by the electric machine which is supplied by the battery set. For instance in a parallel hybrid configuration, such a maximum possible extent may preferably correspond to a significant part of the total hybrid traction power, say 80% or more, versus a small amount of power delivered by the internal combustion engine, say less than 20%. In a series-hybrid vehicle, this may imply that the electric motor(s) retrieve(s) its energy from the storage device rather than from the electric generator.
The high rate depleting phase ends when the state of charge of the battery set reaches the sustaining range, thus triggering the sustaining phase. In the sustaining phase, the actual battery state of charge oscillates within the narrow sustaining range, with the battery set sometimes recovering small amounts of energy from the electric generator and sometimes delivering small amounts of energy to the electric traction motor. Typically, during the sustaining phase, the state of charge of the battery set will be maintained between 40 percent and 60 percent of its total capacity. In case where the vehicle mission is relatively long, the sustaining phase lasts longer than the high rate depleting phase.
This prior art controlling method makes it possible to somewhat downsize the internal combustion engine. Indeed, the electric traction motor can boost the total hybrid traction power and thus compensate for the power loss resulting from this downsizing.
This prior art controlling method which is common to many vehicles is not desirable from many standpoints.
First of all, the prior art controlling method induces a big difference in the way the vehicle behaves during the two main phases, i.e. during the high rate depleting phase and the sustaining phase. Indeed, the performance, the noise and the emissions of the vehicle will be dramatically different between those two main phases.
Besides, controlling a battery set with a short but strong depleting phase followed by a long sustaining phase may shorten the service life of the storage device.
It therefore appears that, from several standpoints, there is room for improvement in the field of plug-in hybrid vehicles.
It is desirable to provide a controlling method avoiding the afore-mentioned disadvantages.
Accordingly, one subject-matter of an aspect of the present invention is a method for controlling a hybrid traction assembly of a hybrid vehicle, the hybrid traction assembly comprising at least:
an internal combustion engine;
at least one electric machine which can be mechanically coupled to the driving wheels so as to perform traction of the hybrid vehicle, the at least one electric machine being suitable for generating electric regenerative energy when the hybrid vehicle slows down;
a storage device suitable for delivering electric traction energy to the electric machine and for recovering electric regenerative energy from the electric generator, wherein the storage device can be connected to an external source of electric power so as to charge the storage device at a high state of charge before a beginning of a vehicle mission;
wherein the method comprises:
an initial depleting phase which begins at the beginning of the vehicle mission and where the hybrid traction assembly is controlled to cause depletion of the storage device at a first mean depletion rate, the first mean depletion rate being a high depletion rate;
at least one sustaining phase where the hybrid traction assembly is controlled so that the state of charge is maintained in a predetermined sustaining range;
the method being characterized in that it further comprises a second depleting phase which extends between the initial depleting phase and the sustaining phase, wherein the hybrid traction assembly is controlled to cause depletion of the storage device at a second mean depletion rate, the second mean depletion rate being lower than the first mean depletion rate.
In the present application, a state of charge (SOC) refers to the state of charge of a battery set, and a mean depletion rate refers to a ratio between a variation of the state of charge over a time variation.
In other words, a controlling method according to the present invention has an overall depletion phase, where the state of charge decreases down to the sustaining range under two successive mean depletion rates, instead of only one high depletion rate for a prior art controlling method.
These and other features and advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as non-limiting examples, embodiments of a hybrid vehicle according to an aspect of the invention and of a controlling method according to an aspect of the invention.